1. Field of the Invention
The present invention relates to a cell flowing ratio controlling method with UPC, i.e., Usage Parameter Control function for monitoring and controlling an allowable cell flowing ratio in UNI (user to network interface) or NNI (network to node interface) in a cell switching system, such as ATM (asynchronous Transfer Mode) switch, and a cell switching system using the same.
More particularly, it relates to a cell flowing ratio controlling method in which cells flowing on a plurality of channels are multiplexed and UPC function can be executed in single device for the multiplexed cells.
2. Description of the Related Art
A cell switching system, such as ATM switch, has UPC function on subscriber interfaces connected to subscriber terminals through a transmission path. It is general that the UPC function is provided for each subscriber channel.
It is disadvantage from an economical view point to provide a cell flowing controlling device, hereinafter, which is referred as to UPC device, individually for each interface of a relatively low speed. Therefore, a method for employing single UPC device by multiplexing a plurality of the above-described subscriber interfaces will be considered now.
FIG. 12 shows a structural block diagram of an ATM switching system. The system includes an ATM switch 1, an input channel processing section 2 and an output channel processing section 3. The ATM switch is a self-routing switch, such as a Banyan switch, for routing each cell to a destined output transmission path according to a header information of the header section in each cell.
The input channel processing section 2 terminates transmission paths, i.e., physical channels, linking to subscribers SUB, and monitors the cell flowing ratio. The output channel processing section 3 including a function of interfacing with an output transmission path, is formed of a buffer memory 30 and an operation and maintenance administration section 31.
The above-described method for employing single UPC device 20 by multiplexing cells flowing on the plurality of the above-described physical channels in the input channel processing section 2 will be now considered. In this case, outputs from the plurality of subscriber interfaces I/F are inputted to channel multiplexer 21. The UPC device 20 measures a flowing ratio of cell signals multiplexed in the channel multiplexer 21. An operation and maintenance administration section 23 is also provided on the input channel processing device 2.
A header converter 22 converts a virtual channel identifier (hereinafter, referred as VCI) employed on a transmission path, of the cell passed through the UPC device 20 into a VCI employed in the ATM switch. Switching is performed by selecting a path of the ATM switch 1 according to the converted VCI.
FIG. 13 is a diagram for explaining an example for measuring a flowing ratio of the multiplexed output cells from the above-described channel multiplexer 21 in single UPC device 20 when cell flowing on two physical channels (subscriber lines) (a) and (b) are multiplexed. FIG. 14 is a timing chart corresponding to FIG. 13.
Cell sequences corresponding to the subscriber lines (a) and (b) are expressed as (a) and (b) respectively in FIG. 14. M means an output from the channel multiplexer 21 in FIG. 14. The cells flow on subscriber channels (a) and (b) are independent, each of which speed is different. When both timings of the cells coincide with each other such as the cells (a3) and (b4) shown in FIG. 14, it is required to delay either timing of the cell (a3) or (b4) in the channel multiplexer 21.
In the example shown in FIG. 14, the timing of the cell (a3) is delayed. The UPC device 20 holds a cell sequence of each subscriber line and calculates the cell flowing ratio of the cell sequence. Therefore, an interval T1 between the cells (a2) and (a3 becomes wider and an interval T2 between the cells (a3) and (a4) becomes narrower on the cell sequences, corresponding to the subscriber line (a), separated from an output of the channel multiplexer 21.
Accordingly, fluctuation occurs on cell arrival timings at the input of the UPC device 20. The fluctuation makes limitation of accuracy for the UPC function.
There is a further problem to limit the UPC accuracy when a ratio of the speed after multiplexed to the total speed in interface circuit I/F does not become suitable ratio in addition to the problem that the cell speed becomes different for each subscriber line.
FIG. 15 is an explanatory diagram of the above-described problem. As difference between the speed after multiplexed and the speed in interface circuit I/F does not become a suitable multiple of the speed on the physical channel correctly, null or invalid cells should be inserted for a remaining areas or spaces when inputting to the UPC device 20. In FIG. 15, areas illustrated by oblique lines, i.e., an area between the cells (b1) and (a2) and an area between the cells (b3) and (a4), of the cell sequence (M) after multiplexed are the inserted null cells.
It is apparent from FIG. 15 that fluctuation on the cell sequence in the channel (a) after multiplexed occurs, caused by the inserted null cells.
Additionally, when multiplexing the plurality of channels to input to the single UPC device 20, the UPC function should be performed in speed faster than total of transmission speed of the channels.
However, in the UPC device 20, it is required to perform a floating point calculation to obtain accurately within a wide range from a low speed rate to a high speed rate. A capacity of a memory can be reduced by the floating point calculation. However, it is required to take much time for decimal point calculation. It is one factor for giving a limit to device operation.
Accordingly, it is an object of the present invention to provide an UPC (cell flowing ratio monitoring) device to overcome the above-described problems when cells flowing on a plurality of channels are multiplexed to perform UPC function in the single UPC device 20.